1. Field of the Invention
The present invention relates to a control system for an internal combustion engine and, particularly, to a control system which estimates a fuel property of the fuel in use.
2. Description of the Related Art Japanese Patent Laid-open No. 2005-171818 discloses a control system for a compression ignition internal combustion engine in which a premix combustion is performed. According to the disclosed system, an actual ignition timing of the fuel is detected when the premix combustion is being performed, and a property of the fuel in use is determined according to an ignition timing error and a variation of the ignition timing error. The ignition timing error is a difference between the detected ignition timing and a previously set reference fuel ignition timing.
When determining a fuel property based on a combustion state of the injected fuel, it is preferable to prevent the determination accuracy from deteriorating. To achieve such a goal, a fuel injection amount for the cylinder that is used to estimate the fuel property should be maintained at a constant value. However, it is necessary to increase the fuel injection amount for the other cylinders, that are not used to estimate the fuel property, in order to operate the engine according to a demanded load on the engine. Therefore, the difference between a torque generated by the cylinder used to estimate the fuel property and a torque generated by the other cylinders becomes large and may cause an unpleasant vibration in the engine.
Further, when the estimation of fuel property is performed during an idling condition of the engine, for example, it is necessary to control a fuel injection amount so that the engine rotational speed is kept at a substantially constant speed. For example, when using one cylinder for the estimation of fuel property in the case of a four-cylinder engine, it is necessary to keep the engine rotational speed constant by controlling the fuel injection amount in the other three cylinders. In this case, when the load on the engine is comparatively great (i.e., the engine is operating in a high-load idling condition), such as when an air conditioner driven by the engine is operating or the shift position of the automatic transmission is set to the D-range (i.e., drive range), the difference between the torque generated by the cylinder used to determine the estimation of fuel property and the torque generated by each of the other cylinders becomes large and may cause the unpleasant vibration as described above. FIG. 14 shows a fuel injection amount QINJ of each cylinder when the engine is operating in the high-load idling condition. In the example shown in FIG. 14, the cylinder #1 is used for the estimation of fuel property (the fuel injection amount is fixed), and the fuel injection amount corresponding to each of the other cylinders #2 to #4 is controlled to keep the engine rotational speed at the constant value. As shown in FIG. 14, the vibration synchronized with the combustion cycle (two rotations of the crankshaft) increases if the difference in the fuel injection amount QINJ of each cylinder becomes relatively large.